feat: initial backend setup with Bun, Hono, and Drizzle ORM

- Added documentation for architecture and code plan
- Implemented database schema for users, expenses, settlements, and transactions
- Set up double-entry bookkeeping system with multi-currency support
- Configured Hono web framework with middleware
- Integrated Drizzle ORM for database operations

backend/README.md

@@ -0,0 +1,15 @@
+# backend
+
+To install dependencies:
+
+```bash
+bun install
+```
+
+To run:
+
+```bash
+bun run index.ts
+```
+
+This project was created using `bun init` in bun v1.2.10. [Bun](https://bun.sh) is a fast all-in-one JavaScript runtime.